Micro-sized(1030.3±178.4 nm) and nano-sized(50.4±8.0 nm) Fe3O4 particles have been fabricated through hydrogen thermal reduction of α-Fe2O3 particles synthesized by means of a hydrothermal process.The morphology and microstructure of the micro-sized and the nano-sized Fe3O4 particles were characterized by X-ray diffraction,field-emission gun scanning electron microscopy,transmission electron microscopy and highresolution electron microscopy.The micro-sized Fe3O4 particles exhibit porous structure,while the nano-sized Fe3O4 particles are solid structure.Their electrochemical performance was also evaluated.The nano-sized solid Fe3O4 particles exhibit gradual capacity fading with initial discharge capacity of 1083.1 mAhg-1 and reversible capacity retention of 32.6% over 50 cycles.Interestingly,the micro-sized porous Fe3O4 particles display very stable capacity-cycling behavior,with initial discharge capacity of 887.5 mAhg-1 and charge capacity of 684.4 mAhg-1 at the 50th cycle.Therefore,77.1% of the reversible capacity can be maintained over 50 cycles.The micro-sized porous Fe3O4 particles with facile synthesis,good cycling performance and high capacity retention are promising candidate as anode materials for high energy-density lithium-ion batteries.
This paper studies and compares the effects of pull–pull and 3-point bending cyclic loadings on the mechanical fatigue damage behaviors of a solder joint in a surface-mount electronic package. The comparisons are based on experimental investigations using scanning electron microscopy(SEM) in-situ technology and nonlinear finite element modeling, respectively. The compared results indicate that there are different threshold levels of plastic strain for the initial damage of solder joints under two cyclic applied loads; meanwhile, fatigue crack initiation occurs at different locations, and the accumulation of equivalent plastic strain determines the trend and direction of fatigue crack propagation. In addition, simulation results of the fatigue damage process of solder joints considering a constitutive model of damage initiation criteria for ductile materials and damage evolution based on accumulating inelastic hysteresis energy are identical to the experimental results.The actual fatigue life of the solder joint is almost the same and demonstrates that the FE modeling used in this study can provide an accurate prediction of solder joint fatigue failure.
High density packaging is developing toward miniaturization and integration,which causes many difficulties in designing,manufacturing,and reliability testing.Package-on-Package(PoP) is a promising three-dimensional highdensity packaging method that integrates a chip scale package(CSP) in the top package and a fine-pitch ball grid array (FBGA) in the bottom package.In this paper,in-situ scanning electron microscopy(SEM) observation is carried out to detect the deformation and damage of the PoP structure under three-point bending loading.The results indicate that the cracks occur in the die of the top package,then cause the crack deflection and bridging in the die attaching layer.Furthermore,the mechanical principles are used to analyse the cracking process of the PoP structure based on the multi-layer laminating hypothesis and the theoretical analysis results are found to be in good agreement with the experimental results.
Plastic prestraining was applied to a solder interconnect to introduce internal defects such as dislocations in order to investigate the interaction of dislocations with electromigration damage. Above a critical prestrain, Bi interfacial segregation to the anode, a clear indication of electromigration damage in SnBi solder interconnect, was effectively prevented. Such an inhibiting effect is apparently contrary to the common notion that dislocations often act as fast diffusion paths. It is suggested that the dislocations introduced by plastic prestraining acted as sinks for vacancies in the early stage of the electromigration process, but as the vacancies accumulated at the dislocations, climb of those dislocations prompted recovery of the deformed samples under current stressing, greatly decreasing the density of dislocation and vacancy in the solder, leading to slower diffusion of Bi atoms.
FeeNi films with compositions of Fee75Ni,Fee50Ni,and Fee30Ni were used as under bump metallization(UBM)to evaluate the interfacial reliability of SnAgCu/FeeNi solder joints through ball shear test,high temperature storage,and temperature cycling.The shear strengths for Fee75Ni,Fee50Ni,and Fee30Ni solder joints after reflow were 42.57,53.94 and 53.98 MPa,respectively,which were all satisfied the requirement of industrialization(>34.3 MPa).High temperature storage was conducted at 150,175 and200 C.It was found that higher Fe content in FeeNi layer had the ability to inhibit the mutual diffusion at interface region below 150 C,and the growth speed of intermetallic compound(IMC)decreased with increasing Fe concentration.When stored at 200 C,the IMC thickness reached a limit for all three films after4 days,and some cracks occurred at the interface between IMC and FeeNi layer.The activation energies for the growth of FeSn2on Fee30Ni,Fee50Ni,and Fee75Ni films were calculated as 246,185,and 81 kJ/mol,respectively.Temperature cycling tests revealed that SnAgCu/Fee50Ni solder joint had the lowest failure rate(less than 10%),and had the best interfacial reliability among three compositions.
Hao ZhangQing-Sheng ZhuZhi-Quan LiuLi ZhangHongyan GuoChi-Ming Lai
A dragonfly wing consists of membranes and both longitudinal and cross veins.We observed the microstructure cross-section at several locations in the dragonfly wing using environmental scanning electron microscopy(ESEM).The organic nature of the junction between the vein and the membrane was clearly identifiable.The membrane was divided into two layers,the upper epidermis and the lower epidermis.These layers extend around the sandwich structure vein,and combine with the adjacent membrane at a symmetrical location along the vein.Thus,we defined this as an organic junction between the vein and the membranes. The organic junction is able to form a tight corrugation angle,which dramatically increases both the warping rigidity and the strength of the wing,but not the torsional rigidity.The torsional deformation is primarily controlled by the microstructure of the longitudinal veins,and is based on the relative rotation angle between the epidermal layer and the inner layer of the vein that forms the zigzag section.
Density functional theory was employed to investigate the bismuth segregation at Cu/Cu3Sn(010) interface.Five initial constructions were introduced by adopting the adhesion energy criterion.Among them,the so-called "between-Cu" construction in which the interface Cu atoms of Cu slab locate along Cu-Cu bond direction in Cu3Sn slab was found to be the most energy-favored at an adhesion energy of 1.96J/m2.Based on this construction,five possible segregation sites were examined,and the most likely segregation site was determined with adhesion energy as low as1.06J/m2,which was almost half of the initial one.Comparing with other sites adhesion energies,it was concluded that size effect took a large part in embrittlement.The analyses of atomic structure and electronic density revealed that the slabs shifted away from interfaces due to bismuth segregation,and the atoms around Bi atom were pressed away.This calculated work agreed qualitatively with reported experimental results.
Wetting of microporous Cu layer by liquid Sn resulted in contact angles from 0 to 33 deg., tunable by varying wetting temperature and porous microstructure. The wetting was dominated by the interfacial metallurgical reaction, which can lead to pore closure phenomenon, as the liquid infiltration facilitating the wetting process.
TiO 2 nanowires were synthesized successfully in a large quantity by thermal evaporation using titanium monoxide powder as precursor. X-ray diffraction results showed that all the products were pure rutile phase of TiO 2 . According to microstructural observations, the nanowires have two typical morphologies, a long straight type and a short tortuous type. The straight nanowires were obtained at a wide temperature range of 900-1050 ℃, while the tortuous ones were formed below 900 ℃. Transmission electron microscopy characterization revealed that both the straight and the tortuous nanowires are single-crystal rutile TiO 2 . The preferential growth direction of the nanowires was determined as [110] orientation according to electron diffraction and high-resolution image analyses. The morphological change of TiO 2 nanowires was discussed by considering the different atomic diffusion rates of Ti atoms caused by the phase transformation in Ti substrate at around 900 ℃.
